An avalanche photodiode, which is used for optical communications or the like, is a semiconductor light receiving element including, in addition to a layer for performing photoelectric conversion, a layer for performing avalanche multiplication of photoelectric converted carriers, which achieves enhanced light reception sensitivity; it is required for the avalanche photodiode to have a low dark current property and high reliability.
Most of the avalanche photodiodes are formed of a compound semiconductor and can be roughly classified into a mesa structure and a planar structure according to its structure. The mesa structure is a structure in which a mesa is formed on a substrate and a pn junction is contained in the mesa. However, a breakdown easily occurs on the surface of the mesa periphery. In general, in order to suppress the breakdown, it is designed to suppress the dark current at law level by employing a structure formed with a slope and a structure in which a buried layer as a high resistance part is provided in the mesa outer peripheral region (for example, Patent Document 1).
The planar structure is a structure in which a pn junction is formed by providing a selective diffusion region, but an edge breakdown in the edge part of the pn junction introduces a problem. When a current flows in the edge part, if voltage is increased, the reverse voltage of the pn junction of a light reception section positioned at the center scarcely increases and thus the function as an avalanche photodiode cannot be demonstrated. Thus, for example, a measure such that a high-resistance guard ring is provided by injecting impurities into the edge part is taken (for example, Patent Document 2).
Patent Document 1: JP-A-2002-324911 (FIG. 1)
Patent Document 2: JP-A-7-312442 (pages 4-6, FIGS. 2 and 6)